Sea stars, sand dollars, sea cucumbers and sea urchins are all echinoderms. Echinoderms exhibit radial symmetry. One characteristic of echinoderms is that while they have a top and bottom, they don't really have a right or left side.
The most obvious characteristic of sea stars are their arms. Depending on the species, sea stars usually have between 5 and 40 arms. All northern sea stars have 5 arms.